Synthesis of HMX via Nitrolysis of DPT Catalyzed by Acidic Ionic Liquids
83
Central European Journal of Energetic Materials, 2011, 8(2), 83-91
ISSN 1733-7178
Synthesis of HMX via Nitrolysis of DPT Catalyzed
by Acidic Ionic Liquids
Zhiyong He, Jun Luo, Chunxu Lü*,
School of Chemical Engineering, Nanjing University of Science
and Technology, Nanjing 210094, China
*
E-mail: [email protected]
Rong Xu, Jinshan Li
Institute of Chemical Materials, CAEP, Mianyang 621900, China
Abstract: The direct nitrolysis of DPT to synthesize HMX with ionic liquids
(ILs) as catalysts was investigated. The results showed that [Et3NH] HSO4 was
the best catalyst among 18 ILs used and the yield of HMX was up to 61% against
45% without IL. The ILs could be efficiently recovered by simple distillation and
extraction after reaction without any apparent loss of catalytic activity even after
10 times recycling.
Keywords: DPT, HMX, nitrolysis, ionic liquid
Introduction
1,3,5,7-Tetranitro-1,3,5,7-tetraazacyclooctane (HMX) is one of the most
powerful military explosives [1-3], but its application is limited owing to high
cost. The conventional methods to synthesize HMX involve the nitrolysis of
hexamine (HA) [4], 3,7-dinitro-1,3,5,7-tetraazabicyclo[3,3,1]nonane (DPT)
[5], 1,3,5,7-tetraacetyl-1,3,5,7-tetraazacyclooctane (TAT) [6], 1,5-diacetyl-3,7dinitro-1,3,5,7-tetraazacyclooctane (DADN) [7], etc. Some substrates such as
TAT and DADN can afford HMX in high yields but need three or more steps,
and they are not economical or environmental friendly.
DPT can be obtained with high yield from broadly available and inexpensive
84
Z. He, J. Luo, C. Lü, R. Xu, J. Li
urea via nitrourea and/or dinitrourea [4, 5, 8, 9], and then HMX can be produced
from DPT by direct nitrolysis. This process avoids the use of large excess of
acetic acid and acetic anhydride, so the total cost could be reduced remarkably.
However, traditional nitrolysis of DPT by HNO3–NH4NO3 only gave HMX in
a low yield of 45% even when more than 30 equiv. of HNO3 was used [10].
Some other HNO3–NH4NO3-base systems were also investigated to afford HMX
in moderate or low yields of 60%, 62% and 39% when MgO [11], P2O5 [12],
SO3 [13] were added, respectively. It was well known that better yields could be
obtained with a mixture of acetic acid with acetic anhydride in the presence of
HNO3–NH4NO3 [14, 15]. Unfortunately, these methods inevitably showed some
drawbacks such as environmental pollution and tedious workup.
N
O2 N N
Ionic liquids:
N
N
N NO 2
NO2
N
Ionic liquids
O2 N N
N NO2
NH4 NO 3,HNO3
N
NO2
+
N
N HX
O
+
N
-
HO 3S
SO3 H
+
N H X-
X-
Et 3 NH X28
X=HSO 4-
X=NO3- ,HSO4 -
X=NO 3-,HSO4- ,TsON
N
O
O
n
SO3 H
X-
X=NO 3- ,TsO- ,CF3 COO-
Et 3 N(CH 2) 4SO 3H X27
Caprolactam X25
N
(CH 2) 4SO 3HMim X24
X=NO3 -,TsO -,CF3 COO-
X=NO3 -,TsO-,CF 3COO-
NH2 X
N
(CH2 )4 SO3HPy X23
Hmim X22
+
SO3 H
X-
N
N
SO3 H
2X-
PEG1000-DAIL(X) 226
X=HSO4 -,NO 3-,TsO-
Scheme 1. Nitrolysis of DPT catalyzed by ionic liquids.
Up to now, ionic liquids (ILs) have attracted much attention as
environmentally benign media or catalysts thanks to many interesting properties
such as wide liquid range, negligible vapor pressure, high thermal stability, and
Synthesis of HMX via Nitrolysis of DPT Catalyzed by Acidic Ionic Liquids
85
good solvating ability for a wide range of substrates and catalysts [16-18]. Qiao
et al. developed a silica gel-immobilized Brønsted ionic liquid and used it as
recyclable solid catalyst in the nitration of aromatic compounds, which offered
practical convenience for the separation of product from catalyst [19]. Qian et
al reported the ultrasonic assisted nitrolysis of DAPT in [BMIM]PF6 to achieve
HMX in a high yield of 66.8% [20]. Little attention has been paid to the nitrolysis
of DPT in an acidic ionic liquid based system. Zhi et al reported a new method
to synthesize HMX from DPT via nitrolysis using N2O5/HNO3 catalyzed with
a new dicationic acidic ionic liquid PEG200-DAIL with 64% yield [21]. In this
paper, we investigated the application of a series of acidic ILs in the synthesis
of HMX by nitrolysis of DPT with HNO3 (see Scheme 1).
Materials and Methods
ILs were prepared by reported methods, DPT was prepared from urea
by known method with a purity of more than 99% [4]. Melting points were
determined on a Perkin-Elmer differential scanning calorimeter. The IR spectra
was recorded on a Nicolete spectrometer and expressed in cm-1 (KBr). Proton
NMR was recorded on Bruker DRX300 (500MHz) spectrometer. The HPLC
(Waters) was recorded on a SPD-6AV UV detector and mobile phase was
methanol and water (70:30), velocity of flow was 1mL/min.
Typical nitrolysis procedure: 0.1 g IL was dissolved in 21.9 g nitric acid
(98%) and cooled below 0 °C, 1.9 g ammonium nitrate (AN) was added,
then 2 g DPT was added in parts to keep the temperature below 0 °C, and the
result mixture was let stand at definite temperature for certain time to ensure
complete reaction. The mixture was poured into an additional funnel and added
dropwise into a flask containing 0.3 mL of water over 20 minutes keeping the
reaction temperature between 60-65 °C. The resulting mixture was stirred for
another 20 minutes, then cooled to 40 °C and additional water was added. After
an additional stirring for 10 minutes the mixture was cooled to 20 °C and let
standing for 10 minutes, the solid product was collected by filtration and rinsed
by water, affording pure HMX after crystallization from acetone and drying in
vacuum. IR (KBr, cm-1) 1560, 1462, 1395, 1203, 1144, 1086, 947, 760, 600.
1
H NMR(DMSO-d6), δ: 6.03(s, 8H, CH2).
86
Z. He, J. Luo, C. Lü, R. Xu, J. Li
Results and Discussion
Initially experiments for direct nitrolysis of DPT were carried out in
NH4NO3/HNO3 system in the presence of [(CH2)4SO3HPy]NO3. Reaction
conditions such as the amount of IL and reaction time were investigated at 25 °C,
and the results was illustrated in Figure 1 and Figure 2. It was observed that the
yield of HMX increased up to 58% when catalyzed by [(CH2)4SO3HPy]NO3
while only 45% was obtained without the IL (see Figure 1). However, the yield
decreased with increasing IL concentration. The reduced yields may arise from
an inappropriate acidity created during the nitrolysis reaction. At the same time,
a shorter reaction time of 20 minutes was needed for nitrolysis in [(CH2)4SO3HPy]
NO3 against a minimum 30 minutes for traditional nitrolysis in NH4NO3/HNO3
(see Figure 2). The yield would decrease with prolonged time owing to HMX
decomposition [13]. Thus, the IL [(CH2)4SO3HPy]NO3 proved to be an efficient
catalyst which accelerate the reaction rate.
60
58
56
Yield/%
54
52
50
48
46
44
42
40
0
1
2
3
4
5
6
7
IL loading/%
Figure 1. The effect of amount of ionic liquid on nitrolysis of DPT.
Reaction conditions: 21.9 g HNO3, 2 g DPT, 1.9 g NH4NO3, 25 °C,
30 min.
Synthesis of HMX via Nitrolysis of DPT Catalyzed by Acidic Ionic Liquids
87
60
58
56
Yield /%
54
52
50
48
46
44
42
40
0
5
10
15
20
25
30
35
40
React time /min
Figure 2. The effect of reaction time on nitrolysis of DPT.
Reaction conditions: 21.9 g HNO3, 2 g DPT, 1.9 g NH4NO3, 25 °C,
4 (wt)% of [(CH2)4SO3HPy]NO3
Table 1. The effect of different ionic liquids on the nitrolysis of DPTa
Entry
IL
HMX (g) Melting point °C Yield (%)b
1
0
1.23
274.1 ~ 274.4
45
2
[HMim]NO3
1.47
273.6 ~ 273.9
54
3
[HMim]TsO
1.49
274.1 ~ 274.3
55
4
[HMim]CF3COO
1.47
273.8 ~ 274.0
54
5
[(CH2)4SO3HPy]NO3
1.60
274.2 ~ 274.5
59
6
[(CH2)4SO3HPy]TsO
1.63
274.4 ~ 274.6
60
7
[(CH2)4SO3HPy]CF3COO
1.58
274.1 ~ 274.5
58
8
[(CH2)4SO3HMim]NO3
1.49
274.2 ~ 274.4
55
9
[(CH2)4SO3HMim]TsO
1.52
274.4 ~ 274.5
56
10 [(CH2)4SO3HMim]CF3COO
1.49
274.0 ~ 274.3
55
11
[Et3NH]HSO4
1.66
274.1 ~ 274.4
61
12
[Et3N(CH2)4SO3H]NO3
1.49
274.3 ~ 274.7
55
13
[Et3N(CH2)4SO3H]HSO4
1.49
274.1 ~ 274.4
55
14
[Caprolactam]NO3
1.52
274.0 ~ 274.2
56
15
[Caprolactam]HSO4
1.52
273.9 ~ 274.3
56
16
[Caprolactam]TsO
1.55
274.3 ~ 274.6
57
17
PEG1000-DAIL(NO3)2
1.47
274.1 ~ 274.3
54
18
PEG1000-DAIL(HSO4)2
1.47
274.0 ~ 274.3
54
19
PEG1000-DAIL(TsO)2
1.49
274.2 ~ 274.5
55
a
b
2.0 g DPT, 1.9 g NH4NO3, 21.9 g HNO3, 4 (wt)% of IL, 25 °C/20 min, 65-70 °C/20 min.
Calculated by HPLC.
88
Z. He, J. Luo, C. Lü, R. Xu, J. Li
A series of acidic ionic liquids were then applied as catalysts in the direct
nitrolysis of DPT in NH4NO3/HNO3 mixture (see Table 1). The product yields
(54%~61%) were significantly better when using acidic ionic liquids as compared
to control experiment (45%). The anions of ILs did not show distinct effect on
the catalytic activity for nitrolysis of DPT. However, the cations had a more
direct effect on the yields. The catalytic activities of [(CH2)4SO3HPy]TsO and
[Et3NH]HSO4 were superior than other ILs. Studies on the nitrolysis mechanism
for HMX preparation has shown that esterification (O-nitration) is a competing
side reaction which occurs during the formation of HMX in the nitrolysis of
DPT, and that the bridge C-N bonds are the weakest ones which resulting in the
formation of HMX. The choice may depend on the rapidity and completeness
with which the free hydroxyl group is esterified (see Figure 3). If compound 3
is esterified quickly enough, the straight-chain product 4 would form. However,
ammonium nitrate appears to favor formaldehyde removal (demethylolation)
or hindered esterification, which facilitated the formation of HMX [29]. All the
cations of the ILs used in this research were organic ammoniums, which might
have the similar function of ammonium nitrate. This might be a reason for the
nitrolysis modification affording higher yields against traditional methods.
a
O2N N
N
N
b N NO2
(DPT)
O 2N N
N NO 2
N
CH2
c
NO2
N
O 2N N
e N NO 2
N
d
CH 2OH
O2N N CH2OH
(3)
(1)
O2N N
N NO 2
N
NO2
(RDX)
NO2
N
CH2OH O 2N N
N NO 2
O 2N N
CH2OH
N
(2)
NO2
(HMX)
CH 2ONO 2
N NO 2
CH 2
N NO 2
CH 2
N NO 2
CH 2
N NO 2
CH 2ONO 2
(4)
Figure 3. Nitration of DPT.
Synthesis of HMX via Nitrolysis of DPT Catalyzed by Acidic Ionic Liquids
89
The recoverability and recyclability of the acidic ILs was then studied.
Table 2 lists the results of repeat experiments using [Et3NH]HSO4 as catalysts.
The results indicated that the yield was reproducible.
Table 2. Repeated experiments of the nitrolysisa
Entry Dosage of IL (%)
HMX (g)
Melting point °C
1
4
1.67
274.4 ~ 274.6
2
4
1.66
274.2 ~ 274.3
3
4
1.67
274.3 ~ 274.5
4
4
1.65
274.1 ~ 274.4
5
4
1.66
274.3 ~ 274.5
Yield (%)b
61
61
61
61
61
2.0 g DPT, 1.9 g NH4NO3, 21.9 g HNO3, 4 (wt)% of [Et3NH]HSO4, 25 °C/20 min,
65-70 °C/20 min.
b
Calculated by quantitative HPLC.
a
After reaction, IL was extracted from the filtrate with 50 mL methylene
chloride, and it was recovered after removing methylene chloride. The recovered
IL was reused in the next nitrolysis reaction after loading refresh substrates.
The yield of HMX was unchanged after five recycles of acidic ionic liquid
[Et3NH]HSO4 and then decreased slightly with continued recycling for up to
ten cycles (see Figure 4).
65
60
55
50
45
Yield /%
40
35
30
25
20
15
10
5
0
0
1
2
3
4
5
6
7
8
9
10
The number of reuse of IL
Figure 4. Repeating experiments using recovered [Et3NH]HSO4.
11
90
Z. He, J. Luo, C. Lü, R. Xu, J. Li
Conclusions
An efficient process for the preparation of HMX from DPT in a NH4NO3/HNO3
nitrolysis mixture catalyzed by a series of acidic ionic liquids was demonstrated.
The ionic liquids could be reused for at least 10 cycles reproducibly affording
HMX in yields up to 61%.
Acknowledgment
Financial support from Chinese National Natural Science Foundation
of China – Academy of Engineering Physics (No. 10976014) is gratefully
acknowledged.
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